There are three important factors to consider when selecting a DC motor for an application: speed, torque, and voltage. The speed of the axis of a DC motor applies a voltage to rotate an axis at a proportional speed of rotation. Shaft speed is offered in revolutions or revolutions per minute (RPM). Specifications generally refer to the unloaded speed, which is the maximum speed the motor can reach when no torque is applied.
The output torque is when the axis rotation generates a rotational force called torque and that torque is given in units of force-distance. It can be of two types: starting torque or continuous torque. https://www.mrosupply.com/motors/ac-motors/general-purpose-motors/221425_09600100_leeson/ The torque of a DC motor is proportional to the induction current, in which case we have the torque constant. The importance of the torque constant is certain. At the available voltage, DC motors can be designed to operate at a specific voltage if needed. We should not forget that because the speed depends on the voltage, the power supply may be a limitation if an adequate specification of the DC motor is not made.